"I wish I could turn back the clock and go back to 2010 and stop abuse on the platform by creating a very specific bar for how to behave on the platform," Costolo said. He added: "I take responsibility for not taking the bull by the horns."

That attitude is quite different from the one he expressed last summer, when BuzzFeed published a highly critical article about Twitter's abuse problem and Costolo's role, or lack thereof, in stopping it. The BuzzFeed report described Twitter as having "virtually been optimized to accommodate" abuse because of the company's culture of leaning toward free and open speech on the platform.

In tweets at the time, Costolo called the article and that accusation "total nonsense and laughably false as anybody who would speak on the record would tell you."

"Absurd," he continued. "Shows a lack of understanding of the very basics of how trust and safety works at Twitter. Sensationalist nonsense."

But amid the rise of President Donald Trump, who has used Twitter at times to spread verifiably false information and to call out his opponents by name, Costolo is more introspective about abuse on the social network.

He told the crowd at Upfront he had started to think Twitter could address the abuse problem by having employees manually curate tweets to emphasize authoritative voices and their opinions over hyperbole, Primack reports.